U.S. patent number 7,804,228 [Application Number 11/959,104] was granted by the patent office on 2010-09-28 for composite passive materials for ultrasound transducers.
This patent grant is currently assigned to Boston Scientific Scimed, Inc.. Invention is credited to Alain Sadaka, Jian R. Yuan.
United States Patent |
7,804,228 |
Sadaka , et al. |
September 28, 2010 |
Composite passive materials for ultrasound transducers
Abstract
Provided herein are composite passive layers for ultrasound
transducers having acoustic properties that can be easily tailored
to the needs of the transducer application using current
microfabrication techniques. In an embodiment, a passive layer
comprises metal posts embedded in a polymer matrix or other
material. The acoustic properties of the passive layer depend on
the metal/polymer volume fraction of the passive layer, which can
be easily controlled using current microfabrication techniques,
e.g., integrated circuit (IC) fabrication techniques. Further, the
embedded metal posts provide electrical conduction through the
passive layer allowing electrical connections to be made to an
active element, e.g., piezoelectric element, of the transducer
through the passive layer. Because the embedded metal posts conduct
along one line of direction, they can be used to provide separate
electrical connections to different active elements in a transducer
array through the passive layer.
Inventors: |
Sadaka; Alain (San Jose,
CA), Yuan; Jian R. (Pleasanton, CA) |
Assignee: |
Boston Scientific Scimed, Inc.
(Maple Grove, MN)
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Family
ID: |
40754175 |
Appl.
No.: |
11/959,104 |
Filed: |
December 18, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090156939 A1 |
Jun 18, 2009 |
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Current U.S.
Class: |
310/334; 600/459;
600/457 |
Current CPC
Class: |
G10K
11/02 (20130101); G10K 11/004 (20130101); Y10T
29/42 (20150115) |
Current International
Class: |
H01L
41/04 (20060101) |
Field of
Search: |
;310/334
;600/457,459 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0119855 |
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Sep 1984 |
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0137529 |
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Apr 1985 |
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EP |
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0351015 |
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Jan 1990 |
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EP |
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06767742 |
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Oct 1995 |
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EP |
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0697257 |
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Feb 1996 |
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EP |
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60135858 |
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Jul 1985 |
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JP |
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94/09605 |
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Apr 1994 |
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WO |
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2007017776 |
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Feb 2007 |
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WO |
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2008/021325 |
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Feb 2008 |
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WO |
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Primary Examiner: SanMartin; J.
Attorney, Agent or Firm: Frommer Lawrence & Haug LLP
Black; Bruce E.
Claims
What is claimed is:
1. An ultrasound transducer comprising: an active acoustic element;
and a passive layer attached to the active acoustic element, the
passive layer comprising: a layer of photoresist material; and a
plurality of conductive posts embedded within the layer of
photoresist material.
2. The transducer of claim 1, wherein the active acoustic element
comprises a piezoelectric element.
3. The transducer of claim 1, wherein the plurality of conductive
posts are orientated substantially perpendicular to an acoustic
emitting face of the active acoustic element.
4. The transducer of claim 1, wherein the conductive posts comprise
metal posts.
5. The transducer of claim 1, wherein at least one of the
conductive posts extends across a thickness of the passive
layer.
6. The transducer of claim 1, wherein the passive layer forms a
matching layer that acoustically couples ultrasound energy from the
active acoustic element.
7. The transducer of claim 1, further comprising an electrode
deposited on a surface of the passive layer, wherein the electrode
is electrically coupled to the active acoustic element through at
least one of the conductive posts.
8. The transducer of claim 1, wherein the passive layer forms a
backing layer that attenuates ultrasound energy propagation below
the active acoustic element.
9. The transducer of claim 1, wherein the conductive posts are made
of silver or nickel.
10. An ultrasound transducer array comprising: a plurality of
active acoustic elements; and a passive layer attached to the
plurality of active acoustic elements, the passive layer
comprising: a layer of photoresist material; and a plurality of
conductive posts embedded within the layer of photoresist
material.
11. The transducer array of claim 10, wherein the active acoustic
element comprises a piezoelectric element.
12. The transducer array of claim 10, wherein the plurality of
conductive posts are orientated substantially perpendicular to an
acoustic emitting face of the active acoustic element.
13. The transducer array of claim 10, wherein the conductive posts
comprise metal posts.
14. The transducer array of claim 10, wherein at least one of the
conductive posts extends across a thickness of the passive
layer.
15. The transducer array of claim 10, wherein the passive layer
forms a backing layer that attenuates ultrasound energy propagation
below the active acoustic elements.
16. The transducer array of claim 10, further comprising a
plurality of electrodes deposited on a surface of the passive
layer, wherein each of the electrodes is electrically coupled to
one of the active acoustic elements through at least one of the
conductive posts.
17. The transducer array of claim 16, wherein each of the
electrodes is electrically coupled to a different one of the active
acoustic elements.
18. The transducer array of claim 16, further comprising an
integrated circuit (IC) chip electrically coupled to the plurality
of active acoustic elements through at least one of the conductors
and through the plurality of electrodes deposited on the surface of
the passive layer, wherein the IC chip comprises at least one metal
contact pad that is directly coupled to the plurality of electrodes
deposited on the surface of the passive layer.
19. The transducer array of claim 10, further comprising an
integrated circuit (IC) chip electrically coupled to at least one
of the active acoustic elements through at least one of the
conductive posts.
20. The transducer array of claim 19, wherein the IC chip is bonded
to the passive layer, with the passive layer and plurality of
acoustic elements disposed on the IC chip.
21. The transducer array of claim 19, wherein the IC chip comprises
a plurality of electrical contacts, and each one of the electrical
contacts is electrically coupled to a different one of the active
acoustic elements in the transducer array through at least one of
the conductive posts.
22. The transducer array of claim 10, wherein the passive layer
forms a matching layer that acoustically couples ultrasound energy
from the active acoustic elements.
23. The transducer array of claim 10, wherein the passive layer is
a backing layer and the transducer array further comprises: a first
array of electrodes disposed between the passive layer and the
plurality of acoustic elements, wherein the first array of
electrodes are electrically coupled to the conductive posts of the
passive layer; and a second array of electrodes disposed on an
opposite surface of the passive layer from the first array of
electrodes, wherein the second array of electrodes are electrically
coupled to the first array of electrodes through the conductive
posts of the passive layer.
Description
FIELD OF THE INVENTION
The present invention relates to ultrasound transducers, and more
particularly to composite passive materials for ultrasound
transducers.
BACKGROUND INFORMATION
An ultrasound transducer is typically fabricated as a stack of
multiple layers that depend on the application of the transducer.
FIGS. 1a and 1b show typical ultrasound transducers. Each
transducer comprises, from the bottom up, a backing layer 30, a
bottom electrode layer 17, an active element layer (e.g.,
piezoelectric element or PZT) 10, a top electrode layer 13, a
matching layer (or multiple matching layers) 20, and a lens layer
(for focused transducers) 35 and 45. The lens may be a convex lens
35 or a concave lens 45. The backing, matching and lens layers are
all passive materials that are used to improve and optimize the
performance of the transducer. The backing layer is used to
attenuate ultrasound energy propagating from the bottom of the
transducer so that ultrasound emissions are directed from the top
of the transducer and the matching layer is used to enhance
acoustic coupling between the transducer and surrounding
environment. Different transducer designs (different sizes,
frequencies, applications, etc.) require passive materials with
different acoustic properties. Therefore, there is a need for
effective methods to control the acoustic properties of these
materials to deliver consistent performance while maintaining
manufacturability and compliance with processing methods.
A common method to control the properties of passive layers is to
add different fillers in different quantities to an epoxy or
polymer to create a matrix. Common filler materials include
tungsten, alumina, and silver (e.g., in powder form). For example,
silver is used in very high quantities to make an otherwise
insulating epoxy conductive. Tungsten and alumina are used to
control the acoustic impedance of the passive layer by varying the
filler/epoxy matrix density. Although the method of using fillers
has several advantages in terms of flexibility, simplicity and
cost, it also has several drawbacks. This method can only raise the
acoustic impedance up to a certain point after which the epoxy
saturates and will not mix with any additional filler. Also, the
filler can move around in the epoxy before the epoxy is cured,
making it difficult to control the final distribution of the filler
in the epoxy. Another drawback with tungsten and alumina is that
the composite material remains nonconductive. Another drawback is
that changing the composition of the passive layers in many cases
also affects their manufacturability.
Some of these drawbacks can be overcome by adding more processing
steps or using novel mixing, casting and fabrication techniques.
However, these techniques eliminate the main advantage of using
filer/epoxy matrices, which is simplicity and flexibility.
Therefore, there is a need for passive layers and fabrication
methods that provide high flexibility and manufacturability without
sacrificing performance or cost.
SUMMARY OF THE INVENTION
Provided herein are composite passive layers for ultrasound
transducers having acoustic properties that can be easily tailored
to the needs of the transducer application using current
microfabrication techniques.
In an embodiment, a passive layer comprises metal posts embedded in
a polymer matrix or other material. The acoustic properties of the
passive layer depend on the metal/polymer volume fraction of the
passive layer, which can be easily controlled using current
microfabrication techniques, e.g., integrated circuit (IC)
fabrication techniques. Further, the metal posts provide electrical
conduction through the passive layer allowing electrical
connections to be made to an active element, e.g., piezoelectric
element, of the transducer through the passive layer. Because the
embedded metal posts in the example embodiment conduct along one
line of direction, they can be used to provide separate electrical
connections to different active elements in a transducer array
through the passive layer.
In an embodiment, a passive layer is fabricated by applying a
photoresist, e.g., using spin coating. Spin coating allows the
thickness of the photoresist to be precisely controlled by varying
the viscosity of the photoresist and spin parameters. The
photoresist is then exposed to UV light through a mask to transfer
a pattern from the mask to the photoresist. Portions of the
photoresist are then selectively removed, e.g., using a developer,
based on the pattern. Metal is then deposited in the areas where
the photoresist has been removed to form the metal posts of the
passive layer. Because the spacing, arrangement, and dimensions of
the metal posts can be precisely controlled by the mask pattern,
this fabrication method allows the metal/polymer fraction volume,
and hence acoustic properties of the passive layer to be easily
controlled.
Other systems, methods, features and advantages of the invention
will be or will become apparent to one with skill in the art upon
examination of the following figures and detailed description. It
is intended that all such additional systems, methods, features and
advantages be included within this description, be within the scope
of the invention, and be protected by the accompanying claims.
BRIEF DESCRIPTION OF THE FIGURES
In order to better appreciate the above recited and other
advantages of the present inventions are objected, a more
particular description of the invention briefly described above
will be rendered by reference to specific embodiments thereof,
which are illustrated in the accompanying drawings. It should be
noted that the components in the figures are not necessarily to
scale, emphasis instead being placed upon illustrating the
principles of the invention. Moreover, in the figures, like
reference numerals designate corresponding parts throughout the
different views. However, like parts do not always have like
reference numerals. Moreover, all illustrations are intended to
convey concepts, where relative sizes, shapes and other detailed
attributes may be illustrated schematically rather than literally
or precisely.
FIG. 1a shows a prior art ultrasound transducer comprising of a
stack of layers with a convex lens.
FIG. 1b shows a prior art ultrasound transducer comprising of a
stack of layers with a convex lens.
FIG. 2 shows a transducer according to an exemplary embodiment of
the present invention.
FIG. 3 shows a transducer according to another exemplary embodiment
of the present invention.
FIG. 4 shows a transducer according to yet another exemplary
embodiment of the present invention.
FIGS. 5a-5e show process steps for fabricating a transducer
according to an exemplary embodiment of the present invention.
FIG. 6 shows a lead connected to a transducer according to an
exemplary embodiment of the present invention.
FIG. 7 shows an exploded view of a transducer array according to an
exemplary embodiment of the present invention.
FIG. 8 shows an exploded view of a transducer array according to
another exemplary embodiment of the present invention.
DETAILED DESCRIPTION
FIG. 2 shows an exemplary ultrasound transducer 105 according to an
embodiment of the invention. The transducer 105 comprises an active
element 110, e.g., a piezoelectric element, and top and bottom
electrodes 113 and 117 deposited on the top and bottom surfaces of
the active element 110, respectively. The electrodes 113 and 117
may comprise thin layers of gold, chrome, or other conductive
material. The transducer's emitting face may have a square shape,
circular shape, or other shape.
The transducer 105 further comprises a matching layer 120 on top of
the active element 110. The matching layer 120 comprises a
plurality of metallic posts 123 embedded in a polymer matrix 127 or
other material. The acoustic properties of the matching layer 120
depend on the metal/polymer volume fraction of the matching layer
120. Generally, the acoustic impedance increases for increases in
the volume fraction of metal. For other materials, the acoustic
properties depend on the metal/material volume fraction, where the
material is the material in which the metal posts are embedded. As
discussed below, the metal/polymer volume fraction can be easily
controlled using current microfabrication techniques, e.g., IC and
MEMS fabrication techniques. Because the metal/polymer volume
fraction can be easily controlled, the acoustic properties of the
matching layer 120 can be easily tailored to the needs of the
transducer application using current fabrication techniques. The
transducer 105 also comprises a backing layer 130 underneath the
active element 110.
FIG. 3 shows an exemplary ultrasound transducer 205 according to
another embodiment of the invention. Similar to the previous
embodiment, the transducer 205 comprises an active element 110,
e.g., piezoelectric element, and top and bottom electrodes 113 and
117 deposited on the top and bottom of the active element 110,
respectively. The transducer 205 also comprises a matching layer
220 on top of the active element 110.
The transducer 205 further comprises a backing layer 230 underneath
the active element. The backing layer 230 comprises a plurality of
metallic posts 233 embedded in a polymer matrix 237 or other
material. The acoustic properties of the backing layer 230 depend
on the metal/polymer volume fraction of the backing layer 230,
which can be easily controlled using current microfabrication
techniques, e.g., IC and MEMS fabrication techniques.
FIG. 4 shows an exemplary ultrasound transducer according to yet
another embodiment of the invention. In this embodiment, the
matching layer 320 comprises a plurality of metallic posts 323
embedded in a polymer matrix 327 or other material. Similarly, the
backing layer 330 comprises a plurality of metallic posts 333
embedded in a polymer matrix 337 or other material.
Processing steps for fabricating a transducer according to an
exemplary embodiment will now be given with reference to FIGS.
5(a)-5(e). In this example, a matching layer is fabricated on the
active element. However, it is to be understood that the processing
steps can also be used to fabricate the backing layer or other
passive layers of the transducer.
FIG. 5(a) shows an active element 110, e.g., a piezoelectric
element, with top and bottom electrodes 113 and 117, e.g., gold on
chrome electrodes.
In FIG. 5(b) a layer of light-sensitive polymer or epoxy 427 is
applied on top of the active element 110 using spin or spray
coating. Other coating processes may also be used. In this example,
spin coating is used to apply the layer of light-sensitive polymer
or epoxy 427. The polymer or epoxy may be mixed with precursors and
solvents to obtain a desired thickness. By varying the polymer or
epoxy viscosity and the spin parameters, the coat thickness can be
precisely controlled. Most light-sensitive epoxies and polymers are
known as photoresists (e.g., UV cured epoxies) and they are
classified as either positive or negative based on their response
to light. Positive photoresist becomes weaker and more soluble when
exposed to light while negative photoresist becomes stronger and
less soluble when exposed to light. Photoresists are commonly used
in IC and MEMS fabrication with consistent repeatable results.
In FIG. 5(c), a mask 460, e.g., chrome on glass, is used in
conjunction with light exposure equipment to form a pattern in the
photoresist 427. In this example, the photoresist 427 is positive
and the mask 460 is transparent 462 in areas where the photoresist
427 is to be removed. UV light 465 is filtered through the mask 460
and reaches the underlying photoresist 427. The areas of the
photoresist 427 corresponding to the transparent areas 462 of the
mask 460 are exposed to the UV light 465. For the example of
negative photoresist, the mask would be opaque in areas where the
photoresist is to be removed.
In FIG. 5(d), the areas of the photoresist 427 that were exposed to
light are removed with a developer, e.g., solvent, leaving the
desired pattern imprinted in the photoresist 427. In FIG. 5(e), the
metal posts 423 are deposited on top of the active element 110 in
the areas where the photoresist 427 has been removed. The metal
posts 423 may be deposited using sputtering, electroplating, or
other metal deposition method. The metal may be nickel, silver, or
other conductive material. The photoresist 427 and embedded metal
posts 423 form the matching layer 420.
The acoustic properties of the matching layer 420 depend on the
metal/polymer volume fraction of the matching layer 420. Because
the spacing, arrangement and dimensions of the metal posts 423 can
be tightly controlled using the above process steps, the
metal/polymer fraction can be tightly controlled to obtain the
desired acoustic properties of the matching layer 420 and optimize
the transducer design. The pattern (opaque and transparent areas)
of the mask determines the spacing, arrangement and dimensions of
the metal posts, and hence the metal/polymer volume fraction. The
above process can also be used to fabricate the backing layer to
control the acoustic properties of the backing layer, and other
passive layers to control their acoustic properties.
Therefore, the above process provides an effective method to
customize the acoustic properties of passive layers for a
particular transducer application. Further, the above process is
compatible with current fabrication methods, e.g. IC and MEMS
fabrication methods.
Instead of the passive layer comprising the photoresist, the
photoresist may be removed, e.g., stripped off, after the metal
posts are deposited. A polymer or epoxy may then be applied around
the metal post to form the passive layer. For the example of epoxy,
the epoxy may be applied around the metal posts, then cured and
ground down to the desired passive layer thickness.
Other materials may be used to form the posts besides metal,
including nonconductive materials such as oxide, nitride, and the
like. In this example, the acoustic properties of the passive layer
depends on the volume fraction of the post material to the polymer,
e.g., photoresist, in the passive layer.
Metal posts embedded in a polymer matrix not only control the
acoustic properties of the passive layer, but also make the passive
layer conductive along one direction. A conductive passive layer is
advantageous in an ultrasound transducer because it simplifies the
electrical connections of the positive and/or negative leads to the
active element.
FIG. 6 shows an example of a lead 510 electrically connected to the
bottom of the active element 110 through the backing layer 230,
which comprises metal posts 233 embedded in a polymer matrix 237.
In this example, the lead 510 may be connected to the backing layer
230, e.g., by a conductive epoxy or solder 515, or laser fused to
the backing layer. A thin electrode layer 520 may be deposited on
the bottom of the backing layer 230 to facilitate the electrical
connection. The lead 510 may be part of a twisted pair wire or
connected at the other end to a coaxial cable. A lead (not shown)
may similarly be electrically connected to the active element
through the matching layer. Alternatively, a portion of the
matching layer may be removed to expose a small area of the top
electrode 113, and the lead (not shown) connected directly to the
top electrode 113.
Because the metal posts embedded in the polymer matrix are
conductive along one direction (thickness direction), the metal
post can be used to provide separate electrical connections to
different active elements in a transducer array. This is
advantageous over silver based conductive epoxy, which cannot
provide separate electrical connections.
The ability of the metal posts to provide separate electrical
connection in a transducer array is illustrated in FIG. 7. FIG. 7
shows an exploded view of an exemplary transducer array comprising
two concentric active elements 610a and 610b, e.g., piezoelectric
elements PZTs. The transducer array may have more than two active
elements.
The transducer array further comprises two electrodes 617a and 617b
on the bottom of the active elements 610a and 610b, respectively.
The electrodes 617a and 617b are electrically isolated from each
other and may comprise thin layers of gold, chrome, or other metal
deposited on the active elements. The transducer array further
comprises a backing layer 630 comprising metal posts 633a and 633b
embedded in a polymer matrix 637. The metal posts 633b are aligned
with the electrode 617b while the other metal posts 633a are
aligned with the electrode 617a. The number and arrangement of the
metal posts shown in FIG. 7 are exemplary only. The backing layer
630 may comprise any number of posts in different arrangements.
Further, the posts may have different shapes than the ones shown in
FIG. 7.
The transducer array also comprises electrodes 640a and 640b on the
bottom of the backing layer 630. The electrodes 640a and 640b may
be connected to separate leads 650a and 650b, respectively, by
conductive epoxy, solder, or the like. The electrode 640b aligns
with metal posts 633b and electrode 617b while the electrode 640a
aligns with metal posts 633a and electrode 617a. Thus, the
electrode 640b provides an electrical connection to active element
610b through metal posts 633b and electrode 617b while the
electrode 640a provides an electrical connection to active element
610a through metal posts 633a and electrode 617a. Therefore, the
embedded metal posts 633a and 633b enable separate electrical
connections to different active elements 610a and 610b in the
transducer array through the passive layer 630. The same principle
may be applied to the matching layer (not shown in FIG. 7) to
provide separate electrical connections through the matching layer.
The separate electrical connections provided by the metal post
allow the active elements in a transducer array to be independently
controlled and driven.
A passive layer comprising embedded metal posts can be used in
other transducer arrays having different configurations and sizes
depending on the application of the array. Examples of transducer
arrays include linear and annular transducer arrays,
two-dimensional transducer arrays, and the like.
The advantages that transducers arrays provide in performance and
beam manipulation generally come at the price of more complex
electronics and controls for coordinating and driving the separate
elements of the arrays. FIG. 8 shows an exploded view of an
exemplary transducer array, in which electronics for controlling
the elements of the array are provided near the transducer array.
The transducer array in FIG. 8 is similar to the one in FIG. 7
except for an integrated circuit (IC) chip 710 connected to the
bottom electrodes 640a and 640b of the backing layer 630. The IC
chip 710 comprises metal contact pads 720a and 720b that align with
electrodes 640a and 640b, respectively. The electrodes 640a and
640b may be bonded to the metal contact pads 720a and 720b,
respectively, e.g., using solder bumps, to electrically connect the
IC chip 710 to the transducer array. The IC chip 710 also comprises
a metal contact pad 730 to connect the IC chip 710 to an ultrasound
system via a cable, twisted pair wires, or the like. The
electronics of the IC chip 710 may be fabricated on a silicon
substrate using standard CMOS microfabrication techniques.
In this embodiment, the IC chip 710 may contain electronics for
individually controlling and driving the active elements 610a and
610b of the array. For example, the electronics of the IC chip 710
may comprise multiplexers and switches for selectively coupling a
signal to one of the active elements. This advantageously reduces
the number of signals that need to be transmitted over a cable to
and from a remote ultrasound system. The unidirectional conduction
of the metal posts 633b and 633a allow the IC chip to individually
address the active elements 610b and 610a, respectively.
Instead of bonding the IC chip to the transducer array, the IC chip
may be located near the transducer array and connected to the
transducer array, e.g., by wires. For example, the IC chip and
transducer array may be mounted in the same housing next to each
other. The IC chip may also be electrically connected to the
transducer array through metal posts embedded in the matching layer
as an alternative or in addition to the backing layer. Further, the
electronics of the IC chip may include filters and processors for
filtering and processing signals from the transducer array before
sending the signals over a cable to the remote ultrasound
system.
Although metal posts were used in the preferred embodiment to
provide conduction through the passive layer, other conductive
materials may be used for the posts.
In the foregoing specification, the invention has been described
with reference to specific embodiments thereof. It will, however,
be evident that various modifications and changes may be made
thereto without departing from the broader spirit and scope of the
invention. For example, the reader is to understand that the
specific ordering and combination of process actions described
herein is merely illustrative, and the invention can be performed
using different or additional process actions, or a different
combination or ordering of process actions. As a further example,
each feature of one embodiment can be mixed and matched with other
features shown in other embodiments. Additionally and obviously,
features may be added or subtracted as desired. Accordingly, the
invention is not to be restricted except in light of the attached
claims and their equivalents.
* * * * *